Coal liquefaction is the process of converting various ranks of coal, including lignite, subbituminous, and bituminous, to petroleum-like hydrocarbon liquids. These liquids contain less sulfur, nitrogen, and ash than the solid starting material and are also easier to transport and utilize. Advantageously, the resulting hydrocarbon liquids are suitable for utilization as refinery feed stocks for the manufacture of, for example, gasoline; jet, diesel and turbine fuels; heating and fuel oils; and other petro-chemical products that may be utilized to meet commercial, residential and industrial fuel requirements.
The coal liquefaction process involves depolymerizing the coal and increasing the ratio of hydrogen to carbon atoms by a factor of from about two to three times that in the starting coal materials. In direct coal liquefaction (DCL), the hydrogen is chemically added to the coal molecules under conditions of high pressure and temperature.
More specifically, in DCL, the coal is first crushed and slurried. Preferably, the slurry is made utilizing a process-derived solvent. A catalyst is then added and/or the solvent may be hydrogenated to improve hydrogen transfer to the coal. The slurry is then heated to, for example, 400.degree.-500.degree. C. and pressurized under hydrogen to a pressure of 140-300 atm. As the coal is heated to the desired reaction temperature and held there for the necessary residence time, chemical bonds are broken and free-radical fragments are generated. These free-radical fragments may then participate in various secondary reactions by, for example, reacting further to form lighter liquids, polymerizing to form high-molecular weight compounds or combining with hydrogen to produce stable liquids. By controlling these secondary reactions it is possible to achieve high conversion rates and good selectively to produce desired, distillable liquid products.
The same basic procedures may be used to liquify other hydrocarbon-based solids such as waste plastics and rubber tires and to coliquefy mixtures of coal with such waste materials. The catalysts described in this patent can also be used for these processes. For the sake of simplicity, the liquefaction process for all such materials will be generally referred to as "direct coal liquefaction" or DCL.
An important factor in the conversion efficiency and yield of any DCL process is the material utilized to catalyze the process. In order to improve processing efficiency a variety of novel techniques have been utilized in the past to produce highly dispersed iron oxide catalyst precursors. These iron oxide catalyst precursors are often reacted with sulfur during liquefaction to provide a catalyst for DCL processing. Under the high pressure and temperature conditions of the DCL process, many iron oxide catalysts undergo phase transformations to Fe.sub.1-x S. A good DCL iron oxide catalyst precursor must be able to maintain its dispersion and transform to a highly dispersed Fe.sub.1-x S phase under DCL reaction condition.